1. Field of the Invention
The present invention relates to a radiation detector that is appropriate for a radiographic apparatus using X rays or the like and a method for producing a photoconductive layer for recording of the radiation detector.
2. Description of the Related Art
Various kinds of radiographic apparatuses using X rays have been proposed and utilized in the field of radiography for medical diagnosis using X rays (radiation). In the radiographic apparatus, a radiation detector (including a semiconductor as a main element) is used as an X-ray image information recording means, and image signals representing an X-ray image related to a subject are obtained by detecting X rays that have passed through the subject by the radiation detector.
Further, various types of radiation detectors are proposed as the radiation detector that is used in the radiographic apparatus. For example, if the radiation detectors are classified based on charge generation processes, in which X rays are converted into charges (electric charges), there are a photo-conversion-type (indirect-conversion-type) detector, a direct-conversion-type detector and the like. In the photo-conversion-type detector, fluorescence emitted from a phosphor by irradiation with X rays is detected in a photoconductive layer and signal charges are obtained. Then, the signal charges are temporarily stored in a charge storage portion. Further, the stored charges are converted into image signals (electric signals), and the image signals are output. In contrast, in the direct-conversion-type detector, signal charges generated in a photoconductive layer by irradiation with X rays are collected at charge collection electrodes and temporarily stored in a charge storage portion. Then, the stored charges are converted into electric signals, and the electric signals are output.
Alternatively, if the radiation detectors are classified based on charge readout processes for reading out charges stored in the radiation detectors to the outsides of the radiation detectors, there are a light-readout-type detector, a TFT-readout-type detector and the like. In the light-readout-type detector, the charges are read out by irradiating the detector with readout light (electromagnetic wave for readout). In contrast, in the TFT-readout-type detector, the charges are read out by scan-driving a TFT (thin film transistor) connected to the charge storage portion.
In the aforementioned radiation detector, charges corresponding to X-ray energy are generated by irradiating a charge generation layer of the radiation detector with X rays, and the generated charges are read out as electric signals. In the radiation detector, the photoconductive layer functions as a charge generation layer. Conventionally, as the material for the photoconductive layer, amorphous selenium (a-Se), PbO, PbI2, HgI2, BiI3, Cd(Zn)Te and the like have been used.
If the amorphous selenium is used as the material, it is possible to easily increase the size (area) of the photoconductive layer by utilizing a thin-coating formation technique (thin-film formation technique), such as a vacuum vapor-deposition method (vacuum evaporation method). However, a photoconductive layer made of the amorphous selenium tends to include many structural defects because of the characteristic of the amorphous selenium as an amorphous material. Hence, if the amorphous selenium is used, the sensitivity of the layer tends to drop. Therefore, an appropriate amount of impurity is generally added (doping) to improve the performance of the photoconductive layer. For example, in U. S. Patent Application Publication No. 20030223534, amorphous selenium doped with alkali metal at 0.01 to 10 ppm is used as the material for the photoconductive layer for recording. Further, in Japanese Unexamined Patent Publication No. 2001-244492, amorphous selenium doped with Na, as an alkali metal, at 70 ppm is used as the material for the photoconductive layer for recording.
Further, a general method for depositing selenium by evaporation is disclosed in Japanese Unexamined Patent Publication No. 58 (1983)-019471.
Conventionally, it has been considered that amorphous selenium has sufficient hole-transportability (positive-hole-transportability or hole-mobility) because it is a P-type semiconductor and that electron-transportability (electron-mobility) of the amorphous selenium needs to be improved to obtain a radiation detector having a high sensitivity. In U. S. Patent Application Publication No. 20030223534 and Japanese Unexamined Patent Publication No. 2001-244492, the electron-transportability of amorphous selenium was improved by doping the amorphous selenium with sodium. The electron-transportability was improved by an effect of doping that electron trap centers included in charge defects of the amorphous selenium can be reduced.
However, to further improve the sensitivity of the layer, it is necessary to improve not only the electron-transportability but the hole-transportability so that generated carriers are transported to both electrodes. The inventors of the present invention have conducted intensive studies and found out that it is possible to greatly improve the hole-transportability while maintaining the high electron-transportability by setting the coordination number of amorphous selenium at a certain value.